Number of Items per Level
Levels in Hell, Slime Pits, and the Ecumenical Temple are generated without any items on the floor. Other levels contain between 6 and 36 items lying on the floor, chosen as described below; after level 5, there's a very small chance (about 0.2%) to get a very rich level instead, which contains 11-100 items.
Some levels (fixed bottom branch, random mini-vaults...) may contain additional "fixed" items, defined in the vault layout.
Type of Items Generated
Items in the Orcish Mines are always piles of gold; other items are randomly generated with the following weights:
If the item level is below 7 and the generator rolled a staff, a book or a wand, it may randomly change the item type to a potion or a scroll instead. Misc items (elemental evoker, summoning item or crystal ball) are only generated at item level 8 and after with a probability of 0.4% + 0.02% per item level.